Microstrip antennas are relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. There are several types of microstrip antennas the most common of which is the microstrip patch antenna or patch antenna. A patch antenna is a narrowband, wide-beam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate, such as a printed circuit board, with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane. It is relatively easy to print an array of patches on a single substrate using lithographic techniques.
Antenna diversity, is a diversity scheme that uses two or more antennas to improve quality and reliability of transmissions on a wireless link. Antenna diversity may be implemented in a receiving system of the wireless link and/or in the transmitting system of the wireless link. Antenna diversity provides a diversity gain that is measurable e.g. by a signal to noise ratio. A particular antenna diversity scheme is polarization diversity, where a pair of antennas transmits radio signals that are orthogonally polarized.
Polarization of antenna identifies the plane in which the electric field of the antenna varies with respect to the Earth's surface. A linear polarization means that the electric field varies in one direction. The linear polarization can be a horizontal or a vertical polarization depending on the orientation of the antenna. Non-linear polarization means that electric field of the antenna varies in a direction that varies with time. Examples of the non-linear polarizations include circular polarization, where the direction of the electric field varies in the directions of the radius of a circle, and elliptic polarization, where the direction of the electric field varies between the directions of the axes of the ellipse.
Radio signals are orthogonally polarized, when the polarized radio signals may be received separately preferably with only minimal or small interference between the radio signals, e.g. a vertically polarized and a horizontally polarized radio signal.
An omnidirectional antenna has a substantially equal radiation pattern in all directions in the radiation pane of the antenna. In the paper of Saunders, W. K. “On the unity Gain Antenna” edited by E. C. Jordan, Electromagnetic Theory and Antennas Part 2, Pergamon Press, Oxford, 1963, it is proven that a “null free antenna”—meaning an antenna having no zero of radiation field over its radiation sphere, cannot have a pattern linearly polarized in all directions. Accordingly, it is difficult to design an antenna that has a substantially omnidirectional radiation pattern and maintain the linear polarization that would be preferred for polarization diversity in the antenna.